Drugs with opioid receptor binding activity are therapeutically useful for pain and detoxication, and morphine and naloxone are widely used as analgesic and antidote, respectively. But morphine has unwanted side effects such as drug dependency or respiratory depression, and its usage is strictly limited. There is a clear medical need for more efficacious drugs with less side effects. Considerable pharmacological and biochemical studies have suggested that there are at least three major classes of opioid receptors designated .mu., .delta. and .kappa.. While morphine has a binding activity for .mu. receptor, drugs with binding activity for .delta. or .kappa. receptors may be more efficacious and have less side effects.
The object of the present invention is to provide novel cyclic peptide compounds having an excellent opioid receptor binding activity and a pharmaceutical composition comprising the same. Another object of the present invention is to provide processes for producing the novel cyclic peptide compounds.